Fixture

ABSTRACT

A fixture which is intended to be screwed firmly into bone tissue. The fixture includes a generally cylindrical anchoring portion provided with an external screw thread. The anchoring portion includes a cavity which extends from the insertion end of the anchoring portion. A number of slots which connect the cavity with the outside of the anchoring portion extend from the insertion end of the anchoring portion. Each slot is delimited by a leading slot wall and a trailing slot wall, the leading and trailing slot walls rotating in the direction defined by the screw thread when screwing in the anchoring portion. At least the radially outermost part of the trailing slot wall defines an angle with the radial direction and is inclined obliquely forward from within and outward in the direction of rotation. The invention also relates to the use of the inventive fixture for firmly securing a prosthesis.

FIELD OF THE INVENTION

The present invention relates to a so-called fixture which is intendedto be screwed firmly into bone tissue and which is of the kind thatincludes a generally cylindrical anchoring portion that is provided withan external screw thread, which anchoring portion which has a cavitywhich opens out at the insertion end of the anchoring portion, and anumber of through-penetrating slots which extend from said insertionend, wherein each slot connects the cavity with the outer side of theanchoring portion and wherein each slot is delimited by a leading and atrailing slot wall, where the leading and trailing walls relate to thedirection of rotation defined by the screw thread when screwing in thefixture.

BACKGROUND OF THE INVENTION

Fixtures of this kind are used in the field of prosthetic surgery,wherein the prosthesis, parts of a prosthesis or a prosthesis holderis/are anchored in the bone tissue of a person with the aid of one ormore such fixtures.

However, the fixtures according to the present invention are intendedprimarily to be anchored in hollow or tubular bone and can be used, forinstance, in the reconstruction of joints, for instance finger joints orhip joints. The fixtures can also be used to fasten artificial legs orother types of prostheses. However, the invention is not limited to suchapplications. The fixture has an anchoring portion and an applicationportion. The anchoring portion is screwed into a hole predrilled in thebone tissue. The hole is given a diameter which is smaller than theouter diameter of the screw thread of the anchoring portion. Theanchoring portion is self-tapping and thus produces a screw thread inthe wall of the predrilled hole when screwed thereinto, while removingbone fragments. Screwing of the anchoring portion into said hole isfacilitated by the slots, by virtue of the bone fragments being pulledloose from the hole by the edges formed between the outer periphery ofthe anchoring portion and the slots. These bone fragments are conveyedto said cavity through the slots.

An example of such a fixture is described in EP 0 595 782, in which theanchoring end of the fixture is provided with four helically extendingslots. This known fixture has been found highly satisfactory in manyrespects and has provided reliable and positive anchorage of aprosthesis fastened with such a fixture.

However, the known fixture has certain drawbacks. For example, as thefixture is rotated, the bone tissue is worked by those edges that areformed between the periphery of the anchoring portion and the trailingwalls of respective slots. The edge between the radially directed slotwall and said peripheral surface generally defines a right angle. Suchan edge works the bone tissue primarily by rubbing against and tearingloose bone fragments from the wall of the predrilled hole when screwingin the anchoring portion. This results in an unfavourable harsh actionon the bone tissue. Moreover, the bone fragments that are so loosenedare unusually large and of an irregular shape. This makes recovery andhealing of the bone material that collects in the cavity of said fixturedifficult to achieve.

Another drawback with the known fixture is that the radial slots causethe geometry of the anchoring portion to become unfavourable when saidportion is subjected to the compressive radially acting forces createdwhen screwing the anchoring portion into the predrilled hole. Thecombined effect of the rotation of the anchoring portion and of theforces that act radially inwards cause the parts situated between theslots to become distorted as a result of said compression, therewithcausing the fastening portion to be deformed to a non-round shape. Thisreduces the strength of the anchorage. Although this deformation can beavoided by increasing the wall thickness of said portions, this increasein wall thickness will cause the anchoring portion to become stiffer andmake it difficult to achieve the radial elasticity desired to achieve astrong anchorage.

The object of the present invention is to provide an improved type offixture in which the aforedescribed drawbacks are eliminated or at leastreduced. The primary object of the invention is to provide a so-calledfixture with which the bone fragments will be detached favourably whenscrewing in the fixture.

SUMMARY OF THE INVENTION

This object of the invention is achieved by virtue of the trailing slotwall in the anchoring portion defining an angle with the radialdirection and sloping obliquely forwards from within and outwards in thedirection in which the fixture is rotated as it is screwed in, saidangle being defined at least at the radially outer part of the anchoringportion.

As a result of the trailing slot wall being so inclined, there isdefined an acute angle between said wall and the periphery of theanchoring portion. As a result of this acute angle, working of the bonein the wall of the predrilled hole is achieved more effectively and moregently, such that bone fragments will be cut loose rather than beingworn loose as in the case of earlier known fixtures of a correspondingkind. The acute angle thus provide a sharp edge that works in the mannerof scalpel in detaching said bone fragments.

According to one preferred embodiment of the inventive fixture, it isnot only the radially outermost part of said slot wall that defines saidangle, but that the whole of the trailing slot wall defines the sameangle. With regard to the actual working process, it will, per se,suffice for solely the outer parts to be inclined. By causing the wholeof the slot wall to slope, there is also obtained a favourable geometryfor the radial compression of the anchoring portion of the fixture assaid anchoring portion is screwed in. When those parts of the anchoringportion located between the slots are pressed radially inwards, theobliquely extending trailing slot wall will support the leading slotwall, meaning that the anchoring portion will retain its circular shapeeven when radially compressed. The parts situated between the slots maybe made relatively thin, so that the elasticity of said parts willfacilitate radial compression of the anchoring portion. The possibilityof compressing the anchoring portion radially while retaining itscircular shape contributes further towards obtaining a strong and securefastening.

According to another preferred embodiment, the leading slot wall slopesobliquely forwards from within and outwards in the direction in whichthe anchoring portion is rotated as it is screwed in. Because both slotwalls are inclined, the geometrical conditions are improved stillfurther, therewith contributing towards retaining the circular shape ofthe anchoring portion during the radial compression that occurs whenscrewing in said portion.

According to another preferred embodiment of the invention, the angledefined between the trailing slot wall and said radial direction is inthe order of 20°-40° . In this angular range, the angle is sufficientlylarge to provide a good cutting action and sufficiently small to avoidthe risk of fracture or like damage at the cutting edge. In the majorityof applications, it is thought that an angle in the range of 27°-33° ismost suitable, wherewith an angle of about 30° may be the most suitable.

According to another preferred embodiment, the number of slots rangesfrom 3 to 10, preferably from. 5 to 7, suitably six slots. The optimumnumber of slots may vary, primarily depending on the size of thefixture, in other words on the prosthesis application used. The optimumnumber of slots may also depend on other factors. In many instances, afixture that includes about six slots constitutes the best balancebetween the aspects that must be observed in order to achieve favourableconditions for screwing in anchoring portion and also for achieving asecure and durable fastening.

According to another preferred embodiment, the cavity has a circularcross-sectional shape and widens conically in a direction towards theinsertion end. As a result of this conical shape, those parts situatedbetween the slots will have the form of tongues whose thicknessesdecrease towards said insertion end. The flexibility of the tongues thusincreases towards said insertion end so as to achieve the desired radialresilience or springiness of said tongues.

According to another preferred embodiment, the width of the slot at itsradially outer end corresponds to 15-35% of the peripheral distancebetween two slots on the outside of the fixture. This relationshipbetween slot width and slot spacing enables an optimum balance to beachieved between working-technical and strength-technical aspects of thefunction of the fixture.

According to a further preferred embodiment of the invention, thefixture is made of titanium. Although the fixture can be made of anyother suitable material, for instance a polymeric material, compositematerial or other metals, it is preferred to use titanium. Titanium hasmainly been found to possess the particular ability of adhering to bonematerial by so-called osseointegration in the absence of those negativereactions that often occur when foreign material is implanted in bodytissue. On a molecular level, titanium is able to interact with and tointegrate with bone tissue, such as to coalesce with the bone. Thismeans that a titanium fixture will be anchored very securely.

These preferred embodiments of the inventive fixture will be apparentfrom the accompany dependent Claims.

The Invention also relates to the use of the inventive fixture insecuring a prosthesis to bone tissue.

The inventive use provides advantages that correspond to those givenabove with respect to the inventive fixture and the preferredembodiments thereof.

The invention will be described in more detail below with reference toan advantageous embodiment of the inventive fixture and with referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinally sectioned view of a fixture according to theinvention.

FIG. 2 is an end view of the fixture according to FIG. 1 as seen fromthe insertion end.

FIG. 3 is an end view seen from the application end of the fixtureaccording to FIG. 1.

(The figures are drawn to a scale of 5:1).

DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS

FIG. 1 is a longitudinal sectional view of an inventive fixture. Theillustrated fixture is intended for a finger joint and is dimensioned tothis end. The fixture comprises an anchoring portion 1 and anapplication portion 2. The anchoring portion has a length of about 20 mmand the application portion a length of about 10 mm.

The anchoring portion includes an external screw thread 3 which isadapted to allow the applicator to be screwed into a hole predrilled inthe bone tissue. The thread is an M6 thread in the illustrated case. Thepredrilled hole will preferably have a diameter that is slightly smallerthan the inner diameter of the thread, i.e. about 4.5 mm. The thread 3extends along the full length of the anchoring portion 2.

The anchoring portion includes a cavity 5 that extends axially andcentrally in the anchoring portion from the insertion end 4 thereof. Thecavity has a depth of about 12 mm and includes farthest in a shortcylindrical part 6 and a conical part 7 that is situated between saidcylindrical part and said insertion end. The conical part has a lengthof about 10 mm, a smallest diameter of about 3 mm and a largest diameterof about 4.4 mm, corresponding to a cone angle of 40.

The anchoring portion 1 has at its insertion end 4 a beveling 8 whichfacilitates screwing of the anchoring portion 1 into the predrilledhole. The bevelling extends along about 2 mm of the fixture and has anangle of about 15°.

As before mentioned, FIG. 2 is an end view of the fixture as seen fromits insertion end. As will be seen from FIG. 2, six slots 9 are disposedaround the anchoring portion 1 of the fixture. Each slot connects thecavity 5 with the outer surface of the anchoring portion 1 and extendsaxially upwards along the major part of the cavity 5 from the insertionend. This therewith results in the formation of tongues 10 between theslots, where each tongue 10 decreases in thickness in a directiontowards the insertion end 4.

Each slot is defined by two mutually parallel slot walls, that is to saya leading slot wall 13 and a trailing slot wall 12 as seen in thedirection of rotation A when screwing in the anchoring portion. Eachslot slopes forwards and outwards in said rotation direction. The slotwalls 12, 13 thus define an angle α with the radial direction, saidangle α being measured with respect to the intersection of respectiveslot walls with the outer surface of the anchoring portion. The angle αis about 30° in the illustrated case.

A sharp edge 13 is thus formed between the trailing slot wall 12 and theouter portion of the anchoring portion, said edge constituting a cuttingedge when screwing in the anchoring portion. The cutting edge has a topangle of about 60°. Each slot has a width of about 0.5 mm, i.e. thedistance between the slot walls is about 0.5 mm.

FIG. 3 is an end view of the fixture as seen from the application end.The application end may be designed in different ways. In the case ofthe illustrated embodiment, the application end includes a central bore15 having a diameter of 2.5 mm. The upper part of the application endhas an hexagonal shape 16 to which a tool can be applied when screwingin the anchoring portion.

The illustrated fixture is secured in the manner described below.

Firstly, a hole is drilled in the bone tissue to which the fixture shallbe fastened. The diameter of the hole drilled will be smaller than theinner diameter of the screw thread on the anchoring portion 1.

The fixture is then entered into the predrilled hole, facilitated by thebevelling 8 at the inner end 4 of the anchoring portion, and the fixtureis turned in the direction of arrow A with the aid of a turning toolapplied to the hexagonal shape 16. The fixture is then screwed into thehole, therewith tapping a screw thread therein. This is achieved withthe aid of the slots 9, more specifically by the cutting edges 14 ofrespective trailing slot walls 12.

The sharp cutting edge 14 cuts loose chippings from the bone material inthe wall of the predrilled hole, these chippings being transportedthrough the slot 9 and collected in the cavity 5 within the anchoringportion of the fixture. At the same time, the tongues 10 formed betweenthe slots 9 are pressed radially inwards by the pressure exerted fromthe defining wall of the predrilled hole Whereafter, respective slotwalls are pressed closer together so as to reduce the width of theslots. The obliqueness of the slots results in a supportive actiontherebetween. This obliqueness of the slots and said supportive actionenables the anchoring portion or fastening portion to retain itscircular shape when compressed radially.

The chippings cut from the wall of the hole drilled in the bone tissueare collected in the cavity and subsequently contribute in strengtheningthe anchorage in the reformed bone tissue.

1-10. (canceled)
 11. A fixture shaped and configured to be screwedfirmly into bone tissue, said fixture comprising: a generallycylindrical anchoring portion formed with an insertion end and having anexternal screw thread, a cavity which opens out at said insertion end,and a number of through-penetrating slots extending from said insertionend, wherein each slot connects the cavity with the outside of saidanchoring portion and wherein each slot is defined by a leading slotwall and a trailing slot wall where said leading and trailing slot wallsrelate to the direction of rotation defined by said screw thread whenscrewing in the fixture, wherein at least the radially outermost part ofsaid trailing slot wall defines an angle α with the radial direction andslopes obliquely forwardly from within and outwardly in said directionof rotation.
 12. The fixture according to claim 11, wherein the whole ofthe trailing slot wall defines the same angle α.
 13. The fixtureaccording to claim 12, wherein said leading slot wall also slopesobliquely forward from within and outward in said direction of rotation.14. The fixture according to claim 13, wherein said leading and trailingslot walls are parallel with one another.
 15. The fixture according toclaim 11, wherein the angle α is 20°-40° at the radially outer end ofthe trailing slot wall.
 16. The fixture according to claim 12, whereinthe angle α is 20°-40° at the radially outer end of the trailing slotwall.
 17. The fixture according to claim 13, wherein the angle α is20°-40° at the radially outer end of the trailing slot wall.
 18. Thefixture according to claim 14, wherein the angle α is 20°-40° at theradially outer end of the trailing slot wall.
 19. The fixture accordingto claim 11, wherein the angle α is 27°-33° at the radially outer end ofthe trailing slot wall.
 20. The fixture according to claim 12, whereinthe angle α is 27°-33° at the radially outer end of the trailing slotwall.
 21. The fixture according to claim 11, wherein the slots are 3 -10in number.
 22. The fixture according to claim 12, wherein the slots are3-10 in number.
 23. The fixture according to claim 15, wherein the slotsare 3-10 in number.
 24. The fixture according to claim 11, wherein theslots are 5-7 in number.
 25. The fixture according to claim 12, whereinthe slots are 5-7 in number.
 26. The fixture according to claim 15,wherein the slots are 5-7 in number.
 27. The fixture according to claim11, wherein the cavity is circular in cross-section and widens conicallyin a direction toward said insertion end.
 28. The fixture according toclaim 12, wherein the cavity is circular in cross-section and widensconically in a direction toward said insertion end.
 29. The fixtureaccording to claim 13, wherein the cavity is circular in cross-sectionand widens conically in a direction toward said insertion end.
 30. Thefixture according to claim 11, wherein the slot width at the radiallyouter end of said slot corresponds to 15-35% of the peripheral distancebetween two slots on the outside of the fixture.
 31. The fixtureaccording to claim 12, wherein the slot width at the radially outer endof said slot corresponds to 15-35% of the peripheral distance betweentwo slots on the outside of the fixture.
 32. The fixture according toclaim 13, wherein the slot width at the radially outer end of said slotcorresponds to 15-35% of the peripheral distance between two slots onthe outside of the fixture.
 33. The fixture according to claim 27,wherein the slot width at the radially outer end of said slotcorresponds to 15-35% of the peripheral distance between two slots onthe outside of the fixture.
 34. The fixture according to claim 11,wherein that the fixture is made of titanium.
 35. The use of a fixtureaccording to claim 11 for anchoring a prosthesis in bone tissue.